Messages in this thread | | | Date | Tue, 2 Nov 2021 22:48:19 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] static_call,x86: Robustify trampoline patching |
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On Tue, Nov 02, 2021 at 07:18:53PM +0100, Ard Biesheuvel wrote:
> > The range check isn't fundamental to CFI, having a check is the > > important thing AFAIU. > > Agreed. If the call site has a direct branch, it doesn't need the range check.
That, from the earlier email:
| And have the actual indirect callsite look like: | | # r11 - &foo | ALTERNATIVE_2 "cs call __x86_indirect_thunk_r11", | "cs call __x86_indirect_cfi_deadbeef", X86_FEATURE_CFI | "cs call __x86_indirect_ibt_deadbeef", X86_FEATURE_IBT
So the callsite has a direct call to the hash-specific and cfi-type specific thunk, which then does an (indirect) tail-call.
The CFI one does the hash check in the thunk and jumps to the function proper, the IBT one on does it in the landing-pad.
The !CFI one ignore it all and simply does an indirect call (retpoline aided or otherwise) to the function proper -- in which case we can free all the thunks.
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